Conventional vehicle powertrains deliver torque from an internal combustion engine to one or more drive axles. Electric powertrains power the drive axle(s) using motor torque from an electric machine. Hybrid vehicle powertrains selectively deliver torque from the engine and/or electric machine(s) in a particular combination depending on the hybrid operating mode. Output torque from a transmission may be delivered to front, rear, or all drive wheels depending on the vehicle configuration.
In a powertrain having independently-powered drive axles, such as an electric all-wheel drive system delivering motor torque to the front or rear drive axle as needed while an engine independently powers the other drive axle, a driver-requested torque is determined as a function of accelerator pedal angle, force, or travel as a torque line in a two-dimensional space as opposed to a single torque point. The torque line describes the sum of all possible axle torque combinations for a given set of inputs. The driver-requested torque can be realized via either on a single drive axle or divided between drive axles. During normal driving, a controller calculates an optimally efficient torque operating point located on the torque line, and commands an axle torque combination that is sufficient for achieving optimal vehicle performance efficiency.
The torque-generating components of a given vehicle propulsion system also have corresponding torque limits. Such component torque limits are typically based on battery, motor, and power inverter temperatures, motor speeds, battery voltage or electrical current, and other factors. However, during a dynamic driving maneuver, such as during aggressive cornering or hard braking, the particular axle torque combination that would ordinarily achieve optimal fuel efficiency may not provide optimal vehicle dynamics performance, e.g., traction and stability. In other words, it may not always be possible to fully satisfy component torque limits and vehicle chassis dynamic performance requirements for a given driver-requested torque point in a powertrain having independent axle torque sources.